Material reduction mill employing ball charges



June 8, 1954 D. WESTON MATERIAL REDUCTION MILL- E MPLOYING BALL CHARGES4 Sheets-Sheet 1 Filed Dec. 9 1953 FALSE TOE' CRUSHING ZO INVENTOR DAVIDWESTON BV-J ATTORNEYS June 8, 1954 WESTON 2,680,570

MATERIAL REDUCTION MILL EMPLOYING BALL CHARGES Filed Dec. 9, 1953 4Sheets-Sheet 2 GRAVITATIONRL FORCE TE-i- INVENTOR DAVID WESTON BY-JWA Q.-ATTORN vs MATERIAL REDUCTION MILL- EMPLOY Filed Dec. 9, 1953 June 8,1954 WESTON 2,680,570

ING BALL CHARGES 4 Sheets-Sheet 3 FEED MATERIAL ZONE INVENTOR DAVIDWESTON BY-Jdlm k ATTORN YS June 8, 1954 WESTON 2,680,570

MATERIAL REDUC'IYI'ON MILL EMPLOYING BALL CHARGES Filed D60. 9, 1953 4Sheets-Sheet 4 CAPACITY LBS/HOURS FJ 4. %CRITICAL SPEED 5 g 4 Q 2- o. 40E as v S co m 30 25 o a v E 2 2o Lu v G U E Q 30 2a 26 24 22 20 I8 I6INVENTOR F ILL t-" 7 I RPM 0 M DAVID wssrou I ammaa m ATTOR NE YSPatented June 8, 1954 MATERIAL REDUCTION MILL EMPLOYING BALL CHARGESDavid Weston,

2 Claims.

This invention relates to the comminution of materials in materialreduction mills employing a ball charge and more particularly it relatesto upstanding circumferentially crusher bars as will be described inmore detail hereinafter. This application is a continuationin-part of mycopending application Serial No. 234,782, filed July 2, 1951.

Heretofore in the operation of ball mills it has been the generalpractice to employ what has been referred to as a seasoned ball charge;that is to say, a ball charge which is made up of individual balls ofvarying diameters. tion, the balls are gradually reduced in size dueLarge balls are operation of the seasoned been used of 35% and thevolume occupied by the ball charge is frequently as high as 50% increasein volume of the ball charge up to this According to this currentpractice, the material undergoing comminution and the ball charge is ofthe order of from about 45% to about 50% of the mill volume and theproportion of balls in of Mineral Dressing 3d edit. 1948).

In contradistinction to the above described prior practices, myinvention contemplates the use of a relatively small ball chargecomposed of relaballs, the ball charge occupying a maximum of 3% of thevolume of the mill exclusive of voids and the combined volume of theball charge and material charge being not more than a maximum of about32% of the volume of the mill.

This novel type of ball charge is used according Toronto, Ontario,Canada Application December 9, 1953, Serial No.

to the invention in combination with a mill which is provided withhighly upstanding crusher bars which are fairly widely alfordsparticularly With combined dry crushing and grinding mills of the kinddescribed in my copending applications Numbers 175,353, filed July 22d,1950, now abandoned, and 749,131 filed May 20, 1947, now Patent2,555,771, and will for convenience be described in conjunction withthis type of mill. Thus, although the present invention has a broaderapplication, it is preferred to use it in connection with mills of thetype described in my said prior applications, in which mills thediameter of the drum is at least twice the length thereof.

The type of mill to which this invention has particular applicationgenerally comprises a drum arranged for rotation in a substantiallyvertical plane, said drum having two end walls and a cylindrical wall,which latter is provided with a plurality of highly upstandingcircumferentially spaced apart crusher bars mounted around the interiorthereof. Inlet and outlet ports for the charging of feed material andwithdrawing of comminuted material are provided in false trunnions aboutwhich the drum is rotated. Feed is generally fed to the mill through theinlet port by gravity and the mill is kept clean by a current of airwhich is drawn across the mill through the inlet port and out the outletport, which current of air on passing through the mill entrains thefinely divided product of the mill and carries it through the outletport where it is passed through a classifier which separates the heavieroversize particles from the stream, the oversize being allowed to flowback into the mill through the outlet port. The above type of mill isbecoming well known and is used in various embodiments fairly widelythroughout the art. For convenience of reference such type of mill willhereinafter be referred to as a mill of the type described.

In my said prior application Serial No. 234,782 filed July 2, 1951,there is described and claimed a method of operating a mill of the typedescribed which comprises; maintaining within the drum a ball chargeconsisting essentially of large balls occupying at least about 0.35% butnot more than about 3% of the total mill volume exclusive of voids;maintaining the total charge in the mill at from about 20% to about 32%of the mill volume where R. is the internal radius of the drum in feedbetween opposing crusher bar faces. It is the speed at which a particleof material of 3" diameter or smaller within the mill will be carriedcompletely around the mill without falling away from the face of thecrusher bars, or for all intents and purposes from the periphery, thecentrifugal force at speeds greater than the critical speed beingsufiicient to hold these particles firmly against the cylindrical wallof the drum at all times.

When operating at a range of speeds as above set forth, the ball chargeof the present invention produces a greatl enhanced crushing actionwithin the mill which results in a considerable increase in capacity insome instances and in other instances in improved metallurgical results.In addition, however, I have found that the ball charge of the presentinvention may be used in a mill of the type described operated withinother ranges of speed to produce improved results of considerable valuein connection with the milling of certain types of material.

The ball charge of the invention which consists of at least ten ballswhich occupy at least about 0.35% but not more than about 3% of the totamill volume exclusive of voids and consisting essentially of ballsweighing more than four pounds and having a diameter of more than aboutthree inches (preferably about five inches) will produce an entirelydifferent type of action within the mill at speeds of from say about40-60% of critical speed than are produced if the mill is operatedwithin the range of from about 84-90% of critical speed in accordancewith the method claimed in the parent application.

The invention and the theory upon which it is based will be described ingreater detail in the following specification in which reference is hadto the accompanying drawings, in which:

Figure 1 is a vertical cross-section of a charged drum in a mill of thetype described to which feed is being supplied, which will be followedby balls of varying diameters when the drum is rotated at a speed withinthe range of from about 84% to 90% of critical;

Figure 2 is a cross-section similar to that shown in Figure 1illustrating the path balls of a ball charge according to the inventionwhen the mill is operated according to the method of the presentinvention;

Figure 3 is a further cross-section similar to that shown in Figure lbut illustrating what takes place when the total charge within the drumis permitted to exceed the maximum according to the method of thepresent invention;

Figure 4 is a graph showing the relation between critical speed andcapacity obtained when using a ball charge according to the presentinvention on sillimanite-corundum; and

Figure 5 is a graph showing variation in size range of product inrelation to mill speed on the same base as the graph of Figure 4.

Referring now more particularly to the drawings, it will be seen fromFigure 1 that generally speaking the smaller the diameter of the ballsin the ball charge, the further they will be carried upwardly by thedrum before the force of illustrating the paths followed by the eter ofabout 5", all of the centrifugal force which is holding them against theperiphery thereof. The reason for this is that the centre of gravity ofa smal ball will be closer to the cylindrical wall of the drum than willbe the centre of gravity of a large ball and, accordingly, the momentarm of the small balls will be greater and centrifugal force holdingthem to the periphery of the drum will be greater. Accordingly, theinterior of the drum may be theoretically divided into zones; there willbe the cataracting zone in which balls of, for instance, ,4 to 11" indiameter, will be falling freely from point near the top of the drum tostrike the descending side of the cylindrical wall of the drum near thebottom with free fall impact; there will be a cascading zone where ballsof 3" diameter and larger will be rolling down or cascading over the topof the charge eventually to move vertically towards the periphery of thedrum near the bottom thereof; and there will be an abrasion zone withinthe charge near the upwardly moving side of the cylindrical wall of thedrum.

If the mill is operated at between 84% to of the critical speed withtotal charge of 20% to 32% of the mill volume including voids, thecharge itself will assume a position against the upwardly moving side ofthe cylindrical wall of the drum with the toe of the charge beingsituated approximately astride a line drawn verti cally through the axisof the drum, with a false toe of feed material extending slightly pastsaid line. It will be seen moreover, that the path followed by the ballsof generally 3" in diameter and over will as they cascade down throughthe cascade zone cause them to approach the cylindrical wall of the drumsubstantially vertically substantially between the true toe and thefalse toe of the charge.

When the mill is operating in accordance with the method claimed in theaforesaid parent application and the ball charge (in accordance with thepresent invention) consists essentially of balls of relatively largediameter, preferably of a diamthe balls in the charge will follow thesame general path which will be generally curved, substantially asillustrated in Figure 2. In fact, it might be considered that the ballcharge behaves as if groups of the balls were strung together in anumber of chains lying parallel to each other along the length of themill and continuously rotating in the direction of the arrows around thepath illustrated in Figure 2. It will be seen, moreover, from Figure 2that the balls descend into the toe of the charge at a point adjacentthe bottom of the mill at which point the radial centrifugal forceproduced by rot-tion of the mill and the force of gravity are actingtogether. Each ball thus entering the toe of the charge will,accordingly, have a high degree of inertia. As each crusher bar entersthe false toe of the charge, the material in the toe between the crusherbar and a ball of the ball charge which is entering the toe in themanner aforesaid will be crushed between the crusher bar and the ball.The crushing action is similar to the action of a jaw crusher and thecrusher bar may be considered the moving jaw and the ball of the ballcharge may be considered to be the stationary jaw During periods when nofeed is being supplied to the mill, and there is therefore no false toe,the crushing action results from the balls being driven into the truetoe of the charge .by the crusher bars.

gravity overcomes ameter of about 3" tremendous proportions.

In accordance with the foregoing, it will be appreciated thattheoretically there should be a suihcient number of balls present in thecharge to form a series of chains around which the balls are spaced withsuilici nt density that series of balls are arriving in the toe of thecharge at about the same rate as crusher bars are entering the toe ofthe charge. It will further be apof the balls as short as possible. Thiscan be accomplished very simply by arranging to have the charge consistessentially of large balls. the balls become reduced in size due to wearduring operation of the mill, they will progressively travel further andfurther in their generally curved course and it is desirable atintervals to eliminate from the mill balls of a diameter of less thanabout 3". It may, however, in some instances be desirable to retain asmall number of balls of under one inch in diameter because of the workwhich may be done by these balls in free fall impact on the particles ofcharge which are also falling through the cataracting zone of the mill.

Although the specific gravity of the balls apparently has no appreciableeffect upon the path that a ball of any given diameter will followduring operation of the mill, it will, of course, be appreciated thatthe efiectiveness of a ball as an inertia body in the crushing zone willbe increased. if the ball is made heavier. It is, accordingly, preferredaccording to the invention to form the balls of the ball charge from amaterial having a high specific gravity. Tungsten carbide compositions,which have a specific gravity of approximately 14 and are immenselyhard, are ideal substances from which to form ball charges according tothe invention. nately, due to material shortages and high cost, tungstencarbide is not always available at a cost which would enable itseconomical use and, consequently, the balls will generally be formedfrom alloy steel of one sort or another.

Generally speaking, any ball which has a dior more and a weight of aboutfour pounds or more will be useful in a cordance with the invention asan inertia body and, accordingly, in carrying out the method of theinvention it is essential to ensure that the charge consist essentiallyof balls meeting these It has been found, moreover, that .a minimum often balls is generally required to assure the formation of the so-calledchains and effectively produce a continual crushing action of thecharacter described above. In mills of from five to nine feet indiameter, I have found that about five inches in diameter is apractical.

maximum limit for the size of individual balls making up the charge. Inlarger mills, larger balls may be used. It should be remembered,however, that as the size of the mill increases .the crushing forceexerted by each ball also increases and the forces exerted by the ballsreach A six inch iron ball which would weight about 32 pounds in an 18-foot mill where the peripheral speed would be about 14 feet per second,

when operating within the cascade zone do not the range of speedsprescribed according to the invention, would place a force on thecrusher bars capable of exceeding the permissible stress material fromwhich these are made, unless special precautions are taken to increasetheir strength. There does not appear, however, in

dicated by the results of the examples hereinfater set forth.

The above-described principle of operation involves three criticalfactors. First of all, the mill must be rotated at a speed of from 84%to 90% of the critical speed in order The third critical factor in themethod of the present in- It has been found that the drops off markedlyif the total volume of charge exceeds 32%. The reason for this isapparent from a consideration of Figure 3. If the total volume of chargebecomes too large then, even at the high speed of operation according tothe pres ent invention, the toe of the charge extends over past thevertical line drawn through the axis of rotation of the drum in themanner illustrated in Figure 3 and balls which are cascading throughapproach the periphery of the drum in a substantially vertical directionand, consequently, are incapable of with the crusher bars to act asefiicient inertia bodies in the manner set forth. Generally speal 2-eificiency of the mill vention the charge volume including voids isdiameter. The data in the following examples is illustrative of theapplication the reduction of three general namely friable material,medium-tough material and tough material. The results achievedillustrate the advantages of the invention and demonstrate the increasein effectiveness of the crushing action obtained when operating inaccordance with the invention. In the case of friable material, forinstance, which is very readily "broken down, as would be expected, theuse of the invention does not increase the capacity of the mill. In thatcase the advantages derived arise as a result oi the quick initialbreakdown of the feed material in the crushing zone permitting more workto be done on the individual broken down particles in the abrasion zoneto produce a final product within the desired range of particle sizewhich has improved metallurgical characteristics. As would be expected,however, in the reduction of medium-tough material and tough material,the increase in efiiciency of the crushing action obtained results in apronounced increase in the capacity of the mill.

EXAMPLE 1 Friable material The following table of results was compiledfrom the operation of the pilot mill using as feed loosely bondedstandstone:

. Screen Am i gg M111 ialysis Prod- Chemical Analysis Nora-Percentvolume of mill occupied by ball charge 1.0%.

In this case the feed material being loosely bonded sandstone, is veryfriable and it is accordingly to be noted that there is no overallincrease in capacity brought about by the use of a ball charge inaccordance with the invention. a metallurgical point oi view, however,it is noteworthy that there is a considerable reduction of iron oxidecontent of the product of requisite grain size representing in all about:1. 28 92; improvement. In the material treated. the iron oxid occurredchiefly as a finely bonded coating to the silica particles. In breakingdown the feed material. on :kly by means of the enhanced crushing actionusing a ball charge according to the invention the abrasion action inthe abrasion zone is permitted to work on the individual particles for alonger length of time and the result, as exemplified above, is that acleaner product is produced. The sandstone used as a feed in thisexample was intended for use in the manufacture of glass and, as is wellknown, the presence of iron oxide in glass sand is exceedinglydeleterious and each 0.61% of iron oxide in a product lowers the gradeoi the glass sand and materially increases the cost of purification inthe subsequent glass manufacturing process.

EXAMPLE 2 M radium-tough material A medium-tough quartzite was used asfeed From material to the pilot mill, giving the following results:

Conditions of Pilot Mill Run Screen Analysis Max. Feed lllanometerProduct.

Ball Charge l Rate lbs.l Reading, 20+100 hr. ins. None 636 9. 4 56. 5 125" Dia. Steel Balls 340 l 9.4 51.5

Nora-Percent of mill volume occupied by inertia media chargeGold-hearing porphyry ore was used as feed to the pilot mill. and thefollowing results were obtained:

Percent l Screen 82 6 3 Mar. Msnom- Occupie-d 'bylnertia Ball ChargeFeed Rate cter N lbs/hr. Reading None 440 7. 0 39. 3 0. 00 10 5 De.Steel Balls,

Total Weight Risk. 850 7.0 31. 9 0.83 45 3 Dia. Steel Balls,

Total Weight 180 lbs. 720 7. 0 26. i] 0. 82 12 3.7" Die. TungstenCarbide Balls, Total wt. 168 lbs 985 7. 0 43. 0 0. 41

Nora: Lbs. Weight oi 5-inch dia. steel ball 18.

This material is considered as one of the toughest materials to crushand grind. It is noteworthy that the use of inertia bodies according tothe invention does not necessarily result in a greater proportion of theproduct being within the 200 screen analysis size range. The reason forthis is that, using no hall charge the crushing action obtained has alow eiliciency so that a large amount of grinding takes place byabrasion between comparatively large solid surfaces. This accounts forthe fineness of grind obtained without the use of a call charge.

The great increase in the crushing efficiency brought about by thepresence of the ball charge according to the invention quickly reducesthe large size particles of feed and produces a much greater range ofparticle sizes within the range where the air stream will remove them,the result being a much higher overall capacity with a smallerpercentage of the product being within the -2l0 screen analysis sizerange. Measured in terms of surface area produced per unit of time theincrease in the capacity is remarkable.

It will be noted, moreover, in comparing the use of 5 inch steel ballswith the use of 3 inch steel balls, that the 5 inch balls produce abetter result both as regards capacity of the mill and as regards screenanalysis of the products.

lurgical result, the improvement in this respect depending upon thenature of the particular material being reduced and the characteristicsof the product considered desirable in the subsequent treatment of suchmaterial.

If a mill of the type described is operated at the relatively low rateor speed of from about 40 to about 65% of critical speed, a situationoccurs within the mill which involves the charge being successivelycarried around to a point on the upwardly moving side of the drum atwhich sudden slippage occurs causing the charge to fall back to aposition in the bottom of the drum. This action repeats itselfcontinuously at intervals of from one to about five seconds dependingupon the diameter of the mill drum. If a ball charge in accordance withthe present invention is present in the charge, the individual balls,acting as inertia bodies, produce a shattering effect as they meet theoncoming crusher bars as the charge slips back towards the bottom of themill. The result with some materials is a greatly increased capacity inthe coarse size range in the product produced and an appreciableincrease in overall capacity or the mill compared to operation inaccordance with the method claimed in the parent application. In thereduction of certain materials where a relatively coarse product and arelatively fine product is desired with a minimum of intermediate sizes(1. e. a typical differential grind problem) operation of a mill of thetype described containing a ball charge according to the invention iscapable in many cases of producing very valuable results. This will beapparent from the following example where a greatly increased proportionof the product was obtained in the +28 4 mesh size range and an overallincrease in capacity was obtained at the expense of the middle sizeranges.

EXAMPLE i Sillimanite-corundum from surface deposits in South WestAfrica was used as a feed material. This material was desired in a finalproduct in two size ranges, firstly a size range of approximately +28mesh, and secondly as a fine product of 100% l mesh, the idea being toproduce approximately 50% of the product in each of the above-mentionedsize ranges while at the same time producing as little as possible inthe immediate range to reduce the amount of regrinding necessary.

A mill of the type described having nominal dimensions of in diameterand 2' in length was charged with about eight hundred pounds of the feedmaterial together with ten 5" manganese steel balls as inertia bodies.The mill was run at a speed of about 85% of critical speed untilgenerally balanced conditions between the feed and products had beenreached, the load in the mill being maintained at approximately 27% oftotal mill volume inclusive of voids. The mill was then run successivelyat a number of lower speeds and the following results were tabulated:

- operation can be Screen Analysis of Cyclone Product Mill Speed,Percent of g fiyj fgg g g per Hour Percent Percent jf fi fgg pee +4 mesh+28 mesh mesh mesh 313 2. 8 l6. 2 l7. 5 63. 4 312 2. 8 18. 3 22. O 56. 9351 2. 9 21. 3 24. 8 51. 0 387 4. 8 33. l 20. 5 41. 9

The aboveresults are tabulated in Figures 5 and 4, Figure 5 illustratingthe increase in perspeed of the mill is varied and Figure 4 illustratingthe increase in overall capacity as the speed of the mill is reduced. Itwill be observed that in this case at about 53% is an optimum grindhaving regard to capacity and coarseness of the product. At this point,slightly over 40% was in the -'-l +28 mesh range of the +4 mesh forregrind. At there is approximately 40% of the product in the mesh range.At the same time, the capacity of the mill is increased from about 313pounds per hour to 377 pounds per hour.

Examination of the products produced leads to the conclusion thatoperating at these slower speeds with a ball charge according to thepresent invention greatly increases the impact crushing factor whilepractically eliminating the grindthe same time,

Furthermore, it will be observed from a com parison of the resultsachieved at higher speeds tribution in the product.

It is obvious, of course, that the shape of the capacity curve will varywidely with the physical the feed material. With matewhich is ofrelatively uniform hardness and in particular with very hard, toughmaterials, a distinct peak is obtained in the capacity curve within arange of critical speeds of from about 84-90% of critical over whichrange of speeds the action of the ball charge of the invention is aspreviously described and claimed in the parent application, while on theother hand with materials such as sand stone which break down rapidly,the capacity curve will tend to be rather flat as would be expectedhaving regard to the results indicated in Example 1, and in general ithas been found that the most satisfactory achieved within the 84-90% ofcritical speed range. It is desired to point out, however, that the ballcharge of the present invention in combination with a mill of the typedescribed can also be most useful in the solution of differentialgrinding problems on particular materials in which type of operation theaction of the ball charge is very different than it is at the higheroperating speeds.

It will be appreciated, therefore, that the combination of a ball chargeas hereinbefore de- 11 fined with a mill of the type described afiordsin itself a new means for the reduction of materials capable ofaffording a wide variety of useful results under a wide variety ofoperating conditions.

While the above tests were conducted using a ball charge consisting ofballs which were essentially spherical in shape, these being thepreferred form of reduction media, it will be appreciated that thepresent invention contemplates the use of any of the well known shapesof reduc tion media and accordingly it is to be understood that WheneverI refer herein to a bal or a0 a ball charge, I intend to include any ofthe usual reduction media.

What I claim as my invention is:

1. In combination with a combined dry crushing and grinding materialreduction mill of the type comprising a drum having a diameter which isat least twice the length thereof arranged for rotation about ahorizontal axis and having a cylindrical wall and two end walls, saidcylindrical wall having mounted thereon a plurality of circumferentiallyspaced apart highly upstanding crusher bars, a ball charge consisting,during normal operation of the mill, of at least ten balls and occupyingat least about 0.35 but not more than about 3% of the mill volumeexclusive of voids, said ball charg consisting essentially of ballsweighing more than four pounds, and having a diameter of more than about3 inches.

2. The combination defined in claim in which the ball charge consistsessentially of balls of about 5" diameter.

No references cited.

